(a) Field of the Invention
The present invention relates to a zoom lens system and, more particularly, to a zoom lens system with a high vari-focal ratio.
(b) Description of the Prior Art
In recent years, for zoom lens systems, there is a large tendency that the vari-focal ratio is being made higher and higher. Generally, known high vari-focal ratio zoom lens systems are arranged that a position of a wide field angle is included in the vari-focal range and, at the same time, the vari-focal ratio is about 6. Even for such high vari-focal ratio zoom lens systems, it is required to make the lens system convenient for operation and for carrying around and, therefore, difficulties occur in the design of the lens system.
The zoom lens system disclosed in Japanese published unexamined patent application No. 14213/85 is known as a zoom lens system which is arranged to attain the object to make the vari-focal ratio high and to make the lens system compact by overcoming the above-mentioned difficulties. Said known zoom lens system comprises four lens groups respectively having positive refractive power, negative refractive power, positive refractive power, and positive refractive power in the order from the object side and is arranged that a plural number of len groups are used as movable lens groups at the time of zooming. To make the lens system compact, said known zoom lens system is arranged that the second lens group, out of said movable lens groups, is moved toward the object side in a position near the wide position so as to thereby make the diameter of the front lens small and, moreover, refractive powers of respective lens groups are made strong. Besides, a glass material with anomalous dispersion is used in the first lens group in order to thereby prevent aggravation of chromatic aberration and variation of monochromatic aberrations to be caused when the lens system is arranged as described in the above.
However, as the vari-focal range of said known zoom lens system is wide, i.e., as the vari-focal ratio is about 6, large variation of aberrations occurs at the time of zooming, residual chromatic aberration is large, and it is impossible to obtain uniform and favorable performance over the whole vari-focal range.